Disable tripleo-ci-fedora-28-standalone job
Disable tripleo-ci-fedora-28-standalone job

tripleo-ci-fedora-28-standalone is deprecated and should be
disabled in all tripleo repos.

Change-Id: I78e5ba9ba33b00d05e67a0b34a1874e6866a2dc4
Task: https://tree.taiga.io/project/tripleo-ci-board/task/1229

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates